Those skilled in the art are aware of a variety of types of phantoms used to determine the resolution abilities of ultrasound scanners. Most of them involve phantoms containing objects to be detected. For example, one type contains metal or plastic fibers, arranged either individually or in pairs, oriented perpendicular to the scanning plane. An ultrasound scanner is tested by its ability to detect the fibers at all and also by a determination of the least separation between fibers such that they can still be resolved.
S. W. Smith and H. Lopez (1982), "A contrast-detail analysis of diagnostic ultrasound imaging," Med. Phys., 9, 4-12, and S. W. Smith, H. Lopez, and W.J. Bodine, Jr. (1985), "Frequency independent ultrasound contrast-detail analysis," Ultrasound in Med. and Biol., 11, 467-477, describe phantoms consisting of a block of gelatin containing eight long, thin cones or, alternatively, stepped cylinders of gelatin. Both the block of gelatin and the cones within it contain plastic beads or particles that function as ultrasound scatterers. Each cone or stepped cylinder contains such scatterers of a different size and/or concentration relative to the scatterers of the background material so that the scattering level of the background material is different from that of each cone or stepped cylinder. The cones or stepped cylinders extend in a plane held at right angles to the scanning plane, so that the size of the portion of a cone or cylinder within any given scanning slice can be varied from a small size near the tip of the cone or smallest step in the cylinder to a large size near the base of the cone or the largest step in the cylinder.
No attempt is made in the Smith phantoms to approximate the shape of the small lesions commonly being searched for in clinical applications of ultrasound scanning. As a consequence, neither the effect of the width of the slice being scanned nor the effect of the fact that an object to be resolved occupies only a part of the slice being scanned is dealt with. The use of only one cone of any given backscatter coefficient leads to other limitations. For example, statistical fluctuations of the "speckle" or "texture" patterns in ultrasound scanners can easily lead to situations in which a particular simulated lesion is overlooked (a false negative) or, on the other hand, a simulated lesion is thought to have been detected when in fact what is seen is an artifact of the machine (a false positive). The use of only a single object of a given backscatter coefficient increases the opportunities for false negatives. Thus, if only one test object is available to be detected and it is missed because of such statistical fluctuations in speckle or texture patterns, a statistical event may have effectively masked the actual ability of the scanner being tested to resolve such a test object. At the least, multiple scanning of the phantom would be necessary to avoid this source of error. False positives are rendered more likely when the location and number of objects to be resolved are known to the person testing the scanner so that random fluctuations are easier to erroneously identify as a resolved image. Certain terms relating to ultrasound, such as the terms "backscatter coefficient" (used above) and "attenuation coefficient" (see below), are well known to those skilled in the art and are used herein with the same meanings as are assigned to them in Madsen et al. (1984), "Method of data reduction for accurate determination of acoustic backscatter coefficients," J. Acoust. Soc. Am., 76, 913-923.
A different approach to that of the Smith phantoms is seen in the phantom reported by A. Goldstein and W. Clayman (1983), "Particle image-resolution test object," J. Ultrasound Med., 2, 195-209. The Goldstein et al. phantom employes a spatially random distribution of very small ultrasound scatterers distributed within a gel. Significance is attached to variations in the texture pattern of an ultrasound image of the phantom as a function of depth, with the amount of blurring in the image utilized as a measurement of resolution capabilities. In contrast to the Smith phantoms, in the Goldstein et al. phantoms there are no macroscopic variations in scattering particle concentrations, and no attempt is made to visualize an object within the phantom that simulates the sort of object that is to be detected in medical application of the scanning equipment.
In a published abstract, B. M. Spitzer, P. L. Carson, and A. L. Scherzinger (1979), "Evaluation of ultrasonic array image quality," Med. Phys., 6, 350, stated that a "series of test objects consisting of cylindrical and spherical inclusions of non-scattering material in a polymer with approximately liver equivalent scattering has been constructed. Visual determination of acoustic noise levels in the anechoic volumes appear to be a simple, sensitive indicator of clinical image quality on real time arrays." The sizes of the spherical inclusions are not given in the abstract, and the abstract contains no discussion of whether the test objects are useful as a means of testing the ability of a scanner to resolve objects.
The PhD thesis of Mitchell M. Goodsitt (1982), A Three Dimensional Model for Generating the Texture in B-scan Ultrasound Images, Department of Medical Physics, University of Wisconsin, Madison, Wis., includes a discussion of a phantom that essentially was a gel block containing two spheres embedded within the gel that differed from the gel only in scatter coefficient. Small particle scatterers were distributed throughout the gel, but the embedded spheres contained none of them. The larger of the embedded spheres was one half inch in diameter, and the smaller was one quarter inch. The resulting phantom differed from the phantoms of Smith in that the effects of slice width could be studied. However, the diameters of the spheres were so large that the spheres would not tax the resolving ability of any commercial ultrasound scanner then or currently in use for medical diagnostic purposes. Goodsitt et al. (1983), "A three dimensional model for generating the texture in B-scan ultrasound images," Ultrasonic Imaging, 5, 253-279, is based on the Goodsitt thesis and does not extend beyond it.
Madsen et al. (1982), "Anthropormorphic breast phantoms for assessing ultrasonic imaging system performance and for training ultrasonographers, Parts I and II," J. Clin. Ultrasound, 10, 67-75 and 91-100, describe anthropormorphic breast phantoms that include spherical objects embedded within a gel breast shape, one of the objects having the scatter characteristics of a cyst, another having the scatter characteristics of a breast tumor, and so forth. Madsen et al. (1982), "An anthropormorphic ultrasound breast phantom containing intermediate-sized scatterers," Ultrasound in Med. & Biol., 8, 381-392, also describe a breast phantom including spherical objects having scatter coefficients resembling cysts and tumor materials. As with the Goodsitt phantom, these anthropomorphic breast phantoms were not designed to facilitate or be useful in testing the limits of the resolving abilities of ultrasound scanners. The objects were too large, too few in number, and of known location.